1. Field of the Invention
The present invention relates to a current source circuit and an operating method thereof and, in particular, to a current source circuit having a current supply controlled in response to an external signal and an operating method thereof.
2. Description of the Background Art
A current source circuit having a current supply controlled in response to an external signal is conventionally used in various devices, such as a current summing type D/A converter and a sample and hold circuit.
FIG. 22 is a schematic block diagram showing a current summing type D/A converter having conventional current source circuits. Referring to FIG. 22, in a current source circuit 704, a current I flows in from a constant voltage source and the current supply is controlled by a switch 701 indicated functionally in response to an external digital control signal. In a current source circuit 705, a current 2I flows in from the constant voltage source and the current supply is controlled by a switch 702 indicated functionally in response to an external digital control signal. In addition, in a current source circuit 706, a current 4I flows in from the constant voltage source and the current supply is controlled by a switch 703 indicated functionally in response to an external digital control signal. A summation of currents flowing in current source circuits selected in response to externally provided digital signals flows through a load resistor 600 having a resistance value R and a voltage between its terminals is to be fetched as an analog output voltage.
On the other hand, FIG. 23 is a schematic block diagram showing a sample and hold circuit having conventional current source circuits. Referring to FIG. 23, current source circuits 804, 805, 806 and 807 serve as bidirectional bridge drivers for driving a diode bridge 801 by a switching control.
FIG. 24 is a schematic block diagram showing an example of a conventional current source circuit used in a device shown in FIGS. 23 and 24, which is described, for example, in "DATA ACQUISITION AND CONVERSION" by Vivian W-K Shen et al., Digest of Technical Papers of IEEE International Solid-State Circuits Conference, pp. 188-189, February, 1983. The current source circuit is a single output type current source circuit which may or may not supply an output current from an output terminal in response to an externally provided control signal.
Referring to FIG. 24, an output terminal 100 of a current source circuit is connected to the drain of an N channel MOS transistor 902. Transistor 902 has its source connected to the drain of an N channel MOS transistor 901 and transistor 901 has its source grounded. One bias voltage source 921 is connected to the gate of transistor 902. The other bias voltage source 920 is connected to the gate of transistor 901 through a conduction path of an N channel MOS transistor 903. Transistor 903 has its gate connected to a signal line 101a and N channel MOS transistors 904 and 905 have their gates commonly connected to a signal line 101b. Transistor 905 has its drain connected to bias voltage source 921 and its source connected to the source of transistor 902. Transistor 904 has its drain connected to the gate of transistor 901 and its source grounded.
An operation of a circuit shown in FIG. 24 will now be described. When turning on a current source circuit, a potential on signal line 101a becomes an H level and a potential on signal line 101b becomes an L level. Then, transistor 903 is turned on and transistors 904 and 905 are turned off, whereby potentials of bias voltage sources 921 and 920 are applied to gates of transistors 902 and 901, respectively. Both transistors 902 and 901 are turned on, whereby a current flows in transistors 902 and 901 from output terminal 100. In other words, a current flowing into a current source circuit is to be obtained from output terminal 100.
When turning off a current source circuit, a potential on signal line 101a becomes an L level and a potential on signal line 101b becomes an H level. Then, transistor 903 is turned off and transistors 904 and 905 are turned on, whereby gates of transistors 902 and 901 are connected to respective sources. As a result, transistors 902 and 901 are turned off and current flowing into a current source circuit is brought to 0.
FIG. 25 is a circuit diagram showing another example of a conventional current source circuit, which is described, for example, in "An 80-MHz 8-bit CMOS D/A Converter" by T. Miki, et al., IEEE Journal of Solid-State Circuit, Vol. SC-21, No. 6, pp. 983-988, December, 1986.
Referring to FIG. 25, a positive output terminal 100a and a complementary output terminal 100b of a current source are connected to drains of N channel MOS transistors 902a and 902b, respectively. Transistors 902a and 902b have their sources commonly connected to the drain of N channel MOS transistor 901 and transistor 901 has its source grounded. Bias voltage source 921 is connected to gates of N channel MOS transistors 902a and 902b through transmission gates 906a and 906b, respectively. Transistors 902a and 902b have their gates connected to drains of N channel MOS transistors 907a and 907b, respectively, and transistors 907a and 907b have their sources grounded. Another bias voltage source 920 is connected to the gate of transistor 901. Signal line 101a is connected to the gate of an N channel MOS transistor constituting transmission gate 906a, the gate of a P channel MOS transistor constituting transmission gate 906b, and the gate of transistor 907. On the other hand, signal line 101b is connected to the gate of an N channel MOS transistor constituting transmission gate 906b, the gate of a P channel MOS transistor constituting transmission gate 906a, and the gate of transistor 907a.
An operation of a circuit shown in FIG. 25 will now be described. The current source circuit is a current source circuit of a complementary output type obtaining an output current from any one of output terminals 100a or 100b. When trying to obtain a current from output terminal 100a, signal line 101a becomes an H level and 101b an L level. Transmission gate 906a and N channel MOS transistor 907b turned on, transmission gate 906b and N channel MOS transistor 907a are turned off, transistor 902a has its gate applied a potential of bias voltage source 921 and transistor 902b has its gate grounded. As a result, a current flows in transistors 902a and 901 from output terminal 100a and a path from terminal 100b to transistor 902b is rendered non-conductive.
On the other hand, when trying to obtain a current from output terminal 100b, signal line 101a becomes an L level and signal line 101b an H level. Transmission gate 906b and N channel MOS transistor 907a are turned on, transmission gate 906a and N channel MOS transistor 907b turned off, transistor 902b has its gate applied a potential of bias voltage source 921 and transistor 902a has its gate grounded. As a result, a current flows in transistors 902b and 901 from terminal 100b and a path from terminal 100a to transistor 902b is rendered non-conductive.
FIG. 27 is a circuit diagram showing a further example of a conventional current source circuit, which is described, for example, in "a 30-MHz 10-CMOS D/A Converter" by K. Oka et al., IEICE Technical Report, Paper No. ICD 88-6, pp. 39-46. 1988.
Referring to FIG. 27, the source of an N channel MOS transistor 912, the drain of an N channel MOS transistor 911 and the source of an N channel MOS transistor 913 are connected to a common node. Transistor 912 has its gate connected to a bias voltage source 914, transistor 911 has its gate connected to a bias voltage source 915 and transistor 913 has its gate connected to a signal line 101a. Transistor 912 has its drain connected to an output terminal 100, transistor 911 has its source connected to a ground potential and transistor 913 has its drain connected to a voltage source potential V.sub.DD.
An operation of the circuit shown in FIG. 27 will now be described. In the circuit shown in FIG. 27, transistors 912 and 913 having the same conductivity type constitute a differential pair. In general, drain current of transistor 912 is larger than that of transistor 913 when a potential on signal line 101a is lower than that of bias voltage source 914, while drain current of transistor 913 is larger than that of transistor 912 when a potential on signal line 101a is higher than that of bias voltage source 914.
Especially, the case should be considered wherein an amplitude of potential change on signal line 101a is a normal logic amplitude. In such a case, when a potential on signal line 101a becomes an L level, drain current of transistor 911 flows from only transistor 912, whereby a current flows from output terminal 100 to conductive paths of transistors 912 and 911. In other words, a current flowing into the current source circuit is obtained from output terminal 100.
On the other hand, when a potential on signal line 101a becomes an H level, drain current of transistor 911 flows from only transistor 913, whereby a current amount flowing into the current source circuit becomes zero.
There were problems as follows in the conventional current source circuits constituted as described above. In a conventional single output type circuit shown in FIG. 24, transistors 903 and 905 are on/off controlled by signals on signal lines 101a and 101b, whereby a current of transistors 901 and 902 constituting a current path is indirectly controlled. On the other hand, in a conventional complementary output type circuit shown in FIG. 25, transistors 907a and 907b and transmission gates 906a and 906b are on/off controlled by signals on signal lines 101a and 101b, whereby a current of transistors 901, 902a and 902b constituting a current path is indirectly controlled.
More specifically, in conventional current source circuits as mentioned above, since transistors constituting a current path are on/off controlled indirectly, five and nine transistors are needed in a single output type and a complementary output type, respectively. This created a problem that the number of devices constituting a current source circuit increased as a whole.
In a conventional single output type circuit shown in FIG. 27, the number of transistors is reduced to three while two bias voltage sources are still required. This results in insufficient simplification of the circuit structure of the current source circuit.
Furthermore, in the conventional single output type circuit shown in FIG. 27, the transistor 911 is always conductive even when a potential on signal line 101a is at an H level and output current at output terminal 100 is zero. This results in the increased power consumption.